The present invention relates to tunable selective devices based on the transmission and reception of forward magnetostatic bulk waves. These waves are commonly propagated by a magnetic layer deposited on a non-magnetic substrate. The selective characteristics are obtained by forming resonant cavities in the surface of the magnetic layer, which are delimited by gratings of grooves or of filamentary areas exposed to an ionic implantation. The cascade wave coupling of two resonant cavities each provided with a microstrip, makes it possible to transmit, in a selective manner, a high frequency electric signal within a very narrow frequency range. This range may be shifted to establish a tuned condition, by altering the intensity of a magnetic field directed at right angles to the plane of the magnetic layer.
The forward magnetostatic bulk waves have isotropic propagation characteristics in the plane of the magnetic layer and, compared to the magnetostatic surface waves, they offer the advantage of a higher saturation level. If it is intended to produce a tunable selective device by placing a transmitting microstrip and a receiving microstrip in a resonant cavity, it may well be possible to obtain a resonance peak at a given frequency, but the direct coupling established between the microstrips has the result that the insertion losses observed close to resonance are barely greater than those observed at the apex of the resonance peak.
If two resonant cavities delimited by parallel reflectorgratings and each equipped with a microstrip are arranged in cascade, a common mode may be isolated by filtering, so that a single resonance peak may be observed within a substantial range of frequencies. Nevertheless, the insertion losses at either side of this resonance peak have a comparatively small drop from the apex of the resonance peak. This leads to inadequate decoupling between the microstrips for the frequencies differing from the resonance frequency.
The unidirectional character of the propagation of the magnetostatic surface waves makes it possible to secure a more satisfactory reduction of the insertion losses at frequencies outside resonance, but at the cost of a power limitation and of a positive temperature drift, which are more difficult to balance.
In order to reduce the aforesaid disadvantages whilst securing a sensible drop in the insertion losses at frequencies other than at resonance, the invention proposes the application of forward magnetostatic bulk waves whilst exploiting the omnidirectional nature of these waves, to provide a satisfactory level of saturation at low frequencies and greater ease in compensating thermal drift. The device obtained may operate as a filter as well as a loop in an oscillator arrangement.